Field of the Invention
The present invention relates to an image capturing apparatus represented by a digital camera and, more particularly, to a technique for correction of unevenness of exposure caused by external light variation (generally called flicker) deriving from fluorescent light or the like generated at the time of shooting.
Description of the Related Art
When shooting by using an image capturing apparatus under a flicker light source, a photometry operation to be performed before shutter release has a problem that a photometry result is unstable because a photometry sensor is influenced by flicker. In order to solve such a problem, the accumulation time of the photometry sensor is controlled to an integer multiple of the emission cycle of flicker or accumulation is intermittently performed at predetermined time intervals. This makes it possible to perform stable photometry of average brightness even in a flicker environment.
On the other hand, with recent improvements in the sensitivity of digital cameras, the cameras have been able to shoot with high-speed shutters even under an artificial light source which causes flicker. In indoor sports shooting or the like, high shutter speed shooting has a merit of being able to shoot blur-free photographs. On the other hand, in high shutter speed shooting under a flicker light source, images sometimes vary in brightness and color temperature for each frame because of the influence of flicker.
In order to solve such a problem, Japanese Patent Laid-Open Nos. 6-209427 and 2006-222935 have disclosed a technique of reducing the influence of flicker by detecting the flicker and performing exposure at the peak position of the flicker at which variations in brightness are minimum.
However, the technique disclosed in Japanese Patent Laid-Open Nos. 6-209427 and 2006-222935 is based on the premise of performing moving image shooting. In still image shooting, when exposure is performed at a similar peak position, the release time lag increases depending on the timing of flicker. If the user is allowed to select whether to perform peak position shooting, exposure conditions optimal for shooting vary depending on whether flicker peak position shooting is performed.
FIG. 18 shows a general photometry output in a flicker environment caused when a commercial power supply (50 Hz) is used. There is a photometry difference between a photometry value AE_peak obtained at the peak of flicker and an average photometry value AE_ave. For this reason, when performing peak position shooting under the exposure conditions calculated based on AE_ave as described above, the resultant photograph is overexposed by Δ.